1. Field of the Invention
The present invention relates to a surface modifier for metal oxide particles and a method for modifying the surface of metal oxide particles using the same, and more particularly to a surface modifier consisting either an alkylsilanepolyol containing a cyclic alkyl group capable of imparting steric hindrance or a mixture of said alkylsilanepolyol with alkylalkoxysilane, and to a method of modifying the surface of metal oxide particles by coating the surface modifier on the hydrophilic surface of the metal oxide particles through chemical bonding so as to impart hydrophobicity or amphiphilicity (hydrophilicity and hydrophobicity) and reactivity to the surface of the metal oxide particles.
2. Description of the Prior Art
Usually, materials are classified into three categories: organic, inorganic and metal materials.
Currently, in order to improve the fundamental physical properties of each of the different materials, complement the shortcomings, maximize the advantages and realize synergistic effects, different materials are hybridized to form, for example, organic/inorganic composite materials. However, one important consideration in hybridization is the technology for controlling the surface polarity of the different materials. This is because effective dispersion is essential to improve the physical properties of different components through mixing. For this purpose, the development of technologies for hybridizing materials after chemically or electrochemically treating the surface of a substrate, coating the surface or treating the surface with a coupling agent is being actively conducted.
Among materials, inorganic materials receive increasing attention and are used in a wide range of applications, because they have heat resistance, cold resistance, electrical resistance, optical properties, mechanical properties and the like over a wide temperature range compared to other materials and, at the same timer are price competitive. Particularly, because they are porous, and thus have a large internal surface area, they are widely used as adsorbers, packing materials for separation columns, or catalyst carriers.
In addition, inorganic materials such as titanium dioxide (TiO2) or zinc oxide are used as cosmetic materials or electronic materials because they have the ability to block UV light. However, these powders have a shortcoming in that, when they are used in oily cosmetic products or hydrophobic cosmetic products, which do not easily smear due to sweat or water, they will not be effectively dispersed, and thus their properties will not be sufficiently exhibited.
Also, these materials are frequently applied in tire compositions, and the surface modification of silica, which is added as an additive during the manufacture of tires in order to reduce the rolling resistance of tires in response to problems of environmental pollution caused by automobiles, to thus increase the fuel economy and the braking power in water or snow, is of increasing importance. However, inorganic particle silica has a hydroxyl group on the surface thereof, and thus is not easily dispersed in non-polar rubbery composite materials due to the cohesive force thereof. For this reason, to date, a silicone binder, a silica dispersing agent and the like have been separately added to tire tread rubber compositions. However, there are problems in that not only a silane binder, but also a silica dispersing agent, must be separately added, alcohol and water, which are solvents, must be removed after surface treatment, and agglomeration occurs due to the hydrolysis and condensation of the silane binder itself in hydrolysis conditions.
In order to solve a problem in which epoxy molding compounds (EMCs) for protecting semiconductors crack at a temperature higher than 200° C., silica is added to improve the physical properties of EMC. Thus, the effective dispersion of silica which is added in the preparation of EMC is a problem.
As described above, although inorganic oxides are widely used in various applications, it is difficult to use them in combination with other materials due to a problem in which they are not uniformly dispersed in other materials that require hydrophobicity, because the surface thereof consists of a hydroxyl group (—OH), and is thus hydrophilic. Accordingly, there is the need to develop an effective and easy method of modifying the surface of inorganic particles into a hydrophobic surface to allow the particles to be dispersed uniformly, and then inducing chemical bonding.
Methods for rendering the surface of inorganic particles hydrophobic can be diverse depending on the kind of inorganic materials, and methods for the surface modification of inorganic materials that are known to date will now be explained briefly.
In a vapor phase method, low-boiling-point silane with Si—H bond is mainly used, but it has the risk of generating hydrogen gas at high temperature, because it has a silicon-hydrogen (Si—H) bond. In attempts to solve this problem, Korean Patent Laid-Open Publication No. 2004-830 discloses a method of preparing silica directly from a silane-based compound having no silica-hydrogen bond, and Korean Patent Laid-Open Publication No. 2006-128358 discloses a method of modifying a silica surface by heating acrylic silane, and an alkylsilane, having a relatively low boiling point, to a temperature of about 200° C., and spraying the heated materials on the silica surface under pressure. The latter method solves the risk problem, because the silica surface is treated at a relatively low temperature, lower than 200° C., without using a low-boiling-point silane having a silicon-hydrogen bond.
Also, a wet method for increasing dispersion in non-polar rubber composite materials is known. US Patent Publication No. 2005-020323B discloses a wet method of modifying a silica surface to create a hydrophobic surface by dehydrating the silica surface at high temperature, introducing an alkaline earth metal into the end of a hydrophobic polymer, and allowing the polymer to react with the silica. This method is not a method of simply coating the surface of silica, but is a grafting method of inducing covalent bonding, and has an advantage in that the bonds are significantly stable in subsequent processes. However, because an alkaline earth metal such as lithium or sodium is used, impurities remaining after the reaction are likely to reduce bond stability.
As other methods, a method of treating cosmetic particles using, as a surface modifier, a polysilane compound having a fluoroester group was reported (Korean Patent Laid-Open Publication No. 2001-19581). However, this method has shortcomings in that the dispersion of the polysilane compound in pigments is not effective and in that the compound should be used in a larger amount than monomolecular compounds. Also, a method of coating the surface of zinc or a zinc alloy with a trialkoxysilane having a primary alkyl group having 3-5 carbon atoms was reported (Korean Patent Laid-Open Publication No. 2004-59977). However, this method has a problem in that the trialkoxysilane is condensed by itself, because steric hindrance cannot be imparted to the primary alkyl group in a hydrolysis step. In addition, methods of treating the surface of inorganic oxides by self-assembly monolayers (SAMs) of octadecyl chlorosilane, alkyl chlorosilane, glycidoxypropyltrimethoxysilane or the like (Tilman, N., Ulman, A., Penner, T. L. Langmuir 1989, 5, 101; Tripp, C. P., Hair, M. L. Langmuir 1992, 8, 1961; Daniels, M. W., Francis, L. F., J. Col. Int. Sci. 1998, 205).
Although efforts to modify the surface of inorganic materials through various methods as described above have been made, silane compounds used as surface modifiers have been limited mainly to silanes containing a primary alkyl group and tetraalkoxysilanes, and thus there were problems in that unstable silanol groups (Si—OH) are condensed with each other in hydrolysis conditions, or the surface modifier agglomerates before dispersion due to polarity.